Since the early days of powered aviation, aircraft have been, from time to time, troubled by accumulations of ice on component surfaces of the aircraft such as wings and struts, under certain flight conditions. Unchecked, such accumulations can eventually so laden the aircraft with additional weight and so alter the aerofoil configuration of the wings as to precipitate an unflyable condition. A search for means to combat the accumulation of ice under flying conditions has been a continuing one and has resulted in three generally universal approaches to removing accumulated ice, a process known generically as de-icing.
In one form of de-icing, leading edges, that is edges of the aircraft component on which ice accretes and are impinged upon by the air flowing over the aircraft and having a point at which this airflow stagnates, are heated to loosen adhesive forces between accumulating ice and the aircraft component. Once loosened, this ice is generally blown from the aircraft component by the airstream passing over the aircraft. Two methods of heating leading edges have enjoyed significant popularity. In one approach a heating element is placed in the leading edge zone of the aircraft component either by inclusion in a rubber boot applied over the leading edge or by incorporation into the skin structure of the aircraft component. This heating element, typically powered by electrical energy derived from a generating source driven by one or more of the aircraft engines, is switched on and off to provide heat sufficient to loosen accumulating ice. In small aircraft powered typically by one or two engines, a sufficient quantity of electrical power may be unavailable for use in electrical de-icing.
In the other heating approach, gasses at elevated temperature from one or more compression stages of a turbine engine are circulated through leading edges of components such as wings and struts in order to effect a thermal de-icing or anti-icing effect. Employed typically only in aircraft powered by turbine engines, the use of these so-called compressor bleeds or by-pass streams from the aircraft engine turbine can result in reduced fuel economy and a lower turbine power output.
The second commonly employed method for de-icing employs chemicals. In limited situations a chemical has been applied to all or part of the aircraft to depress adhesion forces associated with ice accumulations forming upon an aircraft or to depress the freezing point of water collecting upon surfaces of the aircraft.
The remaining commonly employed method for de-icing is typically termed mechanical de-icing. In the principal commercial mechanical de-icing means, pneumatic de-icing, the leading edge zone of a wing or strut component of an aircraft is covered with a plurality of expandable, generally tube-like structures inflatable employing a pressurized fluid, typically air. Upon inflation, the tubular structures tend to expand substantially the leading edge profile of the wing or strut and crack ice accumulating thereon for dispersal into the airstream passing over the aircraft component. Typically, such tube like structures have been configured to extend substantially parallel to the leading edge of the aircraft component. For aerofoils such as wings and stabilizers, these structures may extend the entire span of the aerofoil. A plurality of tube-like structures frequently are positioned on a wing or strut and typically are configured to be parallel to the leading edge of the wing or strut as by placement of a spanwise succession of tubes spaced in chordwise manner away from the leading edge. The plurality of tubes can provide an ice removal function to the entire leading edge profile of the aerofoil or strut.
Conventionally, pneumatic de-icers are formed from a compound having rubbery or substantially elastic properties. Typically, the material forming tubes on such de-icer structures can expand or stretch by 40% or more during inflation cycles causing a substantial change in the profile of the de-icer (as well as thereby the leading edge) and thereby cracking ice accumulating on the leading edge. At least in part because of the large volume of air required for inflating such highly expandable tubes, the times for inflating such tubes have typically historically averaged between about 2 and about 6 seconds. The distortion engendered in an aerofoil profile by inflation of the tubes can substantially alter the airflow pattern over the aerofoil and can adversely effect lift characteristics of the aerofoil.
The rubber or rubber like materials forming these conventional pneumatic de-icers typically are possessed of a modulus of elasticity of approximately 6900 kPa. Ice, as is well known, is possessed of an elastic modulus enabling typical ice accumulations to adjust to minor changes in contours of surfaces supporting such ice accumulations. The modulus of elasticity for ice is variously reported as being between about 275,000 kPa and about 3,450,000 kPa. The modulus of elasticity of rubber compounds used in conventional de-icers however is substantially different from the modulus of elasticity typically associated with ice accumulations, and the large, 40% or greater expansion undergone by the de-icer during inflation traditionally has functioned to crack or rupture the structure of the ice accumulations thereon allowing such accumulations to be swept away by impinging wind streams.
Ice accumulations, in conforming to minor alterations in the contours of surfaces supporting the ice accumulations do so only somewhat slowly. The phenomenon by which ice accumulations conform to changing contours of support surfaces in some ways may resemble the phenomenon of cold flow in thermoplastic materials. Where the ice accumulations are exposed to extremely rapid but minor deformations, an ice coating cannot accommodate such contour changes sufficiently rapidly and shatters as though struck with a hammer. More recently, it has been discovered that a subjecting leading edges of a wing or a stabilizer to electromechanical induced hammering, such as is shown by U.S. Pat. No. 3,549,964, can assist in removing accumulations of ice on the leading edge. Concern respecting the susceptibility of such leading edges to stress fatigue upon being hammered over extended periods of time as yet have functioned in part to preclude substantial commercial development of such electromechanical hammering schemes.
A means for de-icing a leading edge not requiring the application of electrothermal de-icers and/or not requiring the application of pneumatic de-icers which, during the inflated state, substantially distort the leading edge profile for an extended period of time thereby interfering with the efficient performance of a device associated with the leading edge could find substantial application in industry. Additionally, where such a means for de-icing a leading edge does not pose a significant likelihood for long term structural damage associated with stress or fatigue such as may be associated with electromechanical hammering, such a de-icing means could find substantial commercial utility.